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2) Occasionally a tube will develop a small leak. When air gets into the tube, AND when the high plate voltage is
applied, the air molecules can ionize. The glow of ionized air is quite different from the fluorescence glow above--
ionized air is a strong purple color, almost pink. This color usually appears INSIDE the plate of the tube (though
not always). It does not cling to surfaces, like fluorescence, but appears in the spaces BETWEEN elements. A
tube showing this glow should be replaced right away, since the gas can cause the plate current to run away and
(possibly) damage the amplifier.
D. What is Class A, B, AB, ultralinear, etc?
1. Class A means that the power tube conducts the same amount of current all the time, whether idling or producing
full power. Class A is very inefficient with electricity but usually gives very low distortion.
There are single-ended class-A, or SE, amplifiers. They use one or more tubes in parallel, which are all in phase with
each other. This is commonly used in smaller guitar amps and in exotic high-end amplifiers. Many audiophiles prefer
the SE amplifier, even though it has relatively high levels of even-order distortion. Most 300B high-end amplifiers are
SE. Negative feedback, which can be used to decrease the distortion of an amplifier, is felt by some people to sound
inferior. Most SE amps have no feedback.
Push-pull class-A amplifiers also exist--they use two, four or more tubes (always in pairs) which are driven in opposite
phase to each other. This cancels out the even-order distortion and gives very clean sound. An example of a class-A
push-pull amplifier is the M100S. Push-pull Class A operation usually involves low plate voltages and high plate
currents, compared to Class AB operation below. The high currents might tend to wear out the tube cathodes faster
than in an AB amplifier.
2. Class AB applies only to push-pull amplifiers. It means that when one tube's grid is driven until its plate current cuts
off (stops) completely, the other tube takes over and handles the power output. This gives greater efficiency than Class
A. It also results in increased distortion, unless the amplifier is carefully designed and uses some negative feedback.
3. Class B applies only to push-pull amplifiers in audio; it SOMETIMES applies to RF power amplifiers with one tube. It
is like Class AB, except that the tubes idle at or near zero current. This gives even greater efficiency than Class A or
AB. It also results in increased distortion, unless the amplifier is carefully designed and uses some negative feedback.
If careful design is not undertaken, the result may be crossover distortion, which appears at the midpoint of the output
waveform and has very bad-sounding effects in audio. Most solid-state audio amplifiers use class B, because the
transistors undergo less heat stress when idling.
4. Ultralinear operation was invented by David Hafler and Herbert Keroes in 1951. It uses only beam tetrodes or
pentodes, and special taps on the output transformer. The taps connect to the screen grids of the tubes, causing the
screens to be driven with part of the output signal. This lowers distortion considerably. It is usually seen only in hi-fi
amplifiers that use power tubes such as the 6L6, 6550, EL84 or EL34.
E. Why are different kinds of power supplies used in various tube amplifiers? Why do some use tube
amplifiers? Why do some use tube rectifiers, while others use solid-state rectifiers, while still others have
electronic regulation?
Tube rectifiers are still used in power supplies of some hi-end amps, because the current a tube rectifier can produce
varies somewhat with the load. It is quite different in response from a solid-state rectifier. Many audiophiles also prefer
this classic design for much the same reasons. Also, inexpensive solid-state rectifiers can put "hash" into a power
supply, because of their slow transient capability while charging and recharging a filter capacitor 50/60 times a second.
Special high-speed silicon rectifiers are available at high cost. They are rarely used in products other than a few high-
end amplifiers. Tube rectifiers have faster transient response than most solid-state rectifiers, also making them useful
in some high-end designs.
Regulated DC plate power can be very helpful in a push-pull Class AB amplifier. Because the amp draws greatly
different current when at idle and when delivering full power, a regulated supply "sags" less at full power, producing
better transient response in the amplifier. It is expensive to regulate the high voltages in a tube amplifier, so it is done
only in expensive top-line models. Class A amplifiers have less need for regulation since they draw nearly the same